Journal bearing method employing self-stabilizing, true-tilting pad with abruptly-stepped pocket

ABSTRACT

True-tilting pad of journal bearing has face with border region including engagement surface and defining pocket with longitudinal sidebars, bottom, and abrupt step. The pad has leading and trailing portions, with the face extending longitudinally therebetween. The portions can be positioned upstream and downstream, respectively, relative to rotation direction of rotatable shaft having convex surface. The pad is pivotally supported by pivot and can cooperate with one or more other pads and fluid within housing to hydrodynamically and mechanically support the shaft. The pad tilts free of mechanical engagement with other pads. The leading portion can define an entrance to the pocket and the sidebars can converge along the rotation direction. The pocket hydrodynamically increases pressure on the convex surface of the shaft during rotation, and generates pressure effecting on the face counteraction force about the pivot and upstream relative to the rotation direction in order to counteract spragging force.

This application is a divisional of application Ser. No. 08/828,979,filed Mar. 31, 1997, now U.S. Pat. No. 5,772,335.

TECHNICAL FIELD

This invention relates, in general, to tilting pads and, in particular,to tilting pads of a journal bearing for supporting a convex surface ofa rotatable shaft in a housing holding fluid.

BACKGROUND ART

Hydrodynamic pads cooperate amongst themselves and with oil, or otherliquid or gaseous fluid, in the same housing to form an overall bearingfor a journal or shaft to be rotated within the housing. The shaftcommonly rotates with its axis oriented either vertically orhorizontally. Each hydrodynamic pad typically defines a concave arc onits inner face. Further, this arc faces the convex surface of the shaft.Also, a mechanical pivot on the housing often supports the pad.

Instead of the mechanical pivot, one can support the pad using ahydrostatic pivot. An example of such a hydrostatic pivot can beconfigured in accordance with the disclosure of U.S. Pat. No. 4,059,318to Hollingsworth, Nov. 22, 1977, which is hereby incorporated herein byreference in its entirety.

A conventional tilting pad is an example of a tilting type ofhydrodynamic pad. Furthermore, conventional tilting pads are widelyacknowledged to be the most stable type of hydrodynamic pad. An exampleof an overall bearing configuration for a shaft having horizontal axisorientation could include two lower, tilting pads and one upper,stationary pad, in accordance with the disclosure of U.S. Pat. No.4,597,676 to Vohr et al., Jul. 01, 1986, which is hereby incorporatedherein by reference in its entirety.

Hydrodynamic pads work through a wedge effect in the fluid between thepads and the convex surface of the shaft. Such fluid is oftendescriptively called the squeeze film. This fluid wedge yields ahydrodynamic lift acting on the convex surface of the shaft and directedaway from the arc of the pad. One can recognize occurrences of thiswedge effect in common events such as a person water-skiing or anautomobile tire hydroplaning.

Introducing preload is a usual method for enhancing the fluid wedgeeffect. For example, one can assemble the pads to form a circularbearing having a first radius larger than the radius of a particularshaft to be supported. Then, one can remove material from the leadingand trailing portions, or edges, of the pads so they physically can beassembled more closely around the convex surface of the shaft.Nevertheless, the arc of each individual pad still corresponds to thefirst radius. This allows for the preload, as discussed below.

Considering an instance when the shaft is positioned close to anindividual pad and symmetrically with respect to the arc of that pad,one can understand that the convex surface of the shaft is physicallycloser to the arc of the pad at the center of the arc than at either endof the arc. Analysis of the region from the leading edge to the centerof the arc reveals this arrangement gives rise to a converginghydrodynamic fluid wedge between the convex surface and the arc.

Introducing offset of the pivot with respect to the center of the arclength of the pad is another means of enhancing the fluid wedge effect.Typically, the pivot can be offset longitudinally downstream betweenabout fifty to sixty percent of the arc length. One can employ offset toincrease the fluid wedge effect through modification of the relationshipof moments between the leading and trailing lever arms of the pad.Preload, discussed above, can be combined with offset to furtherincrease the fluid wedge effect.

Tilting by the conventional tilting pad further enhances the wedgeeffect. Namely, the conventional tilting pad desirably accentuates theconverging wedge by permitting the leading edge of the pad to pivot awayfrom the convex surface of the shaft.

The overall bearing, formed by the circular arrangement of the pads, hasan overall radius. As discussed above with respect to preload, the arcof each pad could correspond to a first radius different from thetheoretical overall bearing radius into which the pads are assembled.Eccentricity measures the deviation from an ideal condition in which theaxis of the shaft is collinear with the axis of the overall bearing. Inthis ideal condition, one can say the shaft is centered and experienceszero load. Furthermore, in this ideal condition, the shaft has maximumclearance with respect to the overall bearing.

As one deviates from this ideal condition, into many possible non-idealconditions, by loading the shaft, eccentricity increases. Moreover, theclearance of the shaft with respect to a particular pad of the bearingdecreases. During operation at large loads, the shaft assumes maximaloperating eccentricity, assuming a non-failure/non-contact condition. Atcontact during operation, the eccentricity equals one, so the shaft haszero clearance over the particular pad. This operational failurecondition allows the shaft to undergo forced mechanical engaging betweenits convex surface and the face of the particular pad. Of course, atstart-up and shut-down, that is, before and after operational rotationof the shaft, mechanical contact occurs without operational failure.

Hydrodynamic pads are prevalent in turbo-machinery such as pumps,compressors, and turbines. For instance, consider the case of a turbineblade on its shaft. Here, turbine efficiency is determined, in part, byhow little clearance one must design for the tip of the rotating bladeto pass over the stationary housing. This clearance represents a lossbecause it provides a fluid leakage path. Namely, fluid passing throughthe leakage path makes no positive contribution because it escapes work.This loss is characteristic of all turbo-machinery having some type ofrotating impeller. The performance or effectiveness of the rotationaloperation of the turbo-machinery is strongly inversely proportional tothe amount of clearance the designer must provide for operation of theimpeller or blades mounted on the shaft. So, one desires to minimize therequired operating clearance.

During rotation, the shaft tends to orbit elliptically, as is well-knownin the art. This elliptical orbit further contributes to the amount ofclearance a designer must provide for the operation of the shaft duringits rotation. Accordingly, one desires to minimize both the eccentricityof the shaft position and also the ellipticity of its orbit, duringrotation.

For dynamic considerations, a convenient representation of bearingcharacteristics is by spring and damping coefficients. For a horizontalshaft axis orientation, these are obtained as follows.

First, the equilibrium position to support the given load is establishedby computer solution of the well-known Reynolds equation. Here,horizontal and vertical directions are represented by respective X and Ydirections. Second, a small displacement is applied to the shaft in theX direction. A new solution of Reynolds equation is obtained and theresulting forces in the X and Y directions are produced. The springcoefficients are as follows:

where ΔF_(x) =difference between X forces in the displaced andequilibrium ##EQU1## positions where ΔF_(y) =difference between Y forcesin the displaced and equilibrium positions

where Δy=displacement from equilibrium position in Y direction

K_(xy) =stiffness in X direction due to Y displacement

K_(yy) =stiffness in Y direction due to Y displacement

Third, the shaft is returned to its equilibrium position and a Ydisplacement applied. Next, similar reasoning produces K_(xx) andK_(yx). The damping coefficients D_(ij) are produced in a like manner.Namely, velocities, rather than displacements, in the X and Y directionsare consecutively applied with the shaft in the equilibrium position.So, for most fixed bearing configurations, there are a total of eightcoefficients: four spring (or stiffness) and four damping.

The total force on the shaft is:

    F.sub.i =K.sub.ij X.sub.j +D.sub.ij X.sub.j

F_(i) =force in the i^(th) direction, where repeated subscripts implysummation, for example:

The spring and damping coefficients represent a

    K.sub.ij X.sub.j =K.sub.ix X+K.sub.iy Y

linearization of bearing characteristics. Here, one should determine theequilibrium position accurately because the coefficients are valid onlyabout a small displacement region.

The magnitude of the off-diagonal terms of the spring and dampingcoefficients matrices reflects the degree of cross-coupling in thebearing configuration. One should note that the matrix of the springcoefficients is commonly referred to as the stiffness matrix. Forexample, consider the following common geometrical and operatingconditions of a single-piece, two axial groove bearing for a horizontalshaft.

Shaft Diameter, D=5 in.

Bearing Length, L=5 in.

Active Pad Angle, θ_(p) =160° (10° grooves on either side of pad)

Radial Clearance, c=0.0025 in.

Operating Speed, N=5000 rpm

Lubricant Viscosity, μ=2×10⁻⁶ lb-sec/in.²

Eccentricity Ratio, ε=0.5

Load Direction=Vertical Down

For these conditions, a computer solution yields the following results:

Bearing Load, w=20,780 lbs.

Horsepower Loss, hp=15.51

Minimum Film Thickness, h_(M) =0.00125 in.

Side Leakage, q_(s) =0.941 gpm

Spring and Damping Coefficients: (The signs, positive or negative, ofthe coefficients conform to the rotor dynamic codes utilized.)

Spring Coefficients: (lbs/in.) ##EQU2## Damping Coefficients:(lbs-sec./in.) ##EQU3##

The magnitude of the off-diagonal terms (K_(xy), K_(yx), D_(xy), D_(yx))evidences the above bearing configuration has very strongcross-coupling. That is, the terms off the diagonal of terms (K_(xx),K_(yy), D_(xx), D_(yy)) extending from the upper left to the lower rightpositions in the matrices have large magnitudes.

In one known configuration, conventional tilting pads are interleaved.There, each single pad cannot independently find its own equilibriumposition. Rather, in addition to fluid pressure forces, each pad mustrespond to two other forces on its leading and trailing edges owing tomechanical contact with adjacent pads. In particular, the padexperiences reaction forces on its ends from touching preceding andconsecutive pads. So, the pads do not tilt independently. What a firstpad does influences a second pad, and so on. As introduced above, suchinterdependencies appear mathematically as sizable off-diagonal terms inthe stiffness and damping matrices. Furthermore, sizable cross-couplingterms give rise to destabilizing forces in the bearing, undesirablyproducing the well-known phenomenon of half-speed whirl.

The designing of tilting pads presents many challenges. Commondifficulties, whose effects need minimizing, include unloaded padinstability, pad wiping over the pivot, and hot fluid carryover, whichare discussed below. Conventional tilting pads exhibit many shortcomingsin facing these problems.

For the case of normal steady state rotation of a horizontally-orientedshaft, the pads in a conventional tilting pad bearing assembly situatedabove a horizontal split of the bearing are unloaded. Here, the unloadedpads tend to oscillate about their pivots. Rocking back and forth causesthe leading edge of such a pad to bang the convex surface of the shaft.This phenomenon, commonly known as spragging, can dislodge largesections of babbitt metal from the backing of the conventional tiltingpad. Resultant dangers include separation of the babbitt metal along thelength of the pad as well as carry of the dislodged babbitt metal intoclearance space between the pad and the shaft. This undesirably resultsin metal contact and excessive heating, which in turn lead tooperational failure.

In a conventional tilting pad subject to extremely large loading, forcedmechanical engaging occurs between the convex surface of the shaft andthe arc of the pad. Of course, this wiping usually occurs on the arcportion trapped directly over the pivot, rather than on the freelytilting ends.

Hot fluid carryover is a classic problem inherent in tilting padbearings. Many researchers have devised elaborate configurations fordiverting fluid flow away from the region between the trailing andleading edges of successive tilting pads. In particular, shearing stressin the fluid because of an upstream pad heats the fluid. This heating ofthe fluid is a function of the fluid film thickness, which in turn is afunction of the separation between the shaft and the pad face.Horsepower loss is inversely proportional to this film thickness.

Thus, a need exists for a tilting pad that minimizes the occurrences ofunloaded pad instability and the effects of pad wiping over the pivot aswell as hot fluid carryover. A further need exists for a bearingarrangement that increases hydrodynamic support for the convex surfaceof the shaft and decreases horsepower loss.

SUMMARY OF THE INVENTION

The shortcomings of the prior art are overcome and additional advantagesare provided through the provision of a true-tilting pad having a facethat includes a border region defining a pocket with longitudinalsidebars, a bottom, and an abrupt step. The true-tilting pad also hasleading as well as trailing portions. The leading portion is adapted tobe positioned upstream relative to the rotation direction of a shaft.The trailing portion is adapted to be positioned downstream relative tothe rotation direction of the shaft. The face extends longitudinallybetween the leading and trailing portions. The true-tilting of thepresent invention pad is pivotally supported by a pivot. The borderregion includes an engagement surface. The shaft is adapted to berotated by a prime mover and has a convex surface. The true-tilting padis adapted to cooperate with one or more other pads and fluid within ahousing to hydrodynamically and mechanically support the shaft. Inaddition, the true-tilting pad tilts free of mechanical engagement withthe other pads. The pocket, with its longitudinal sidebars, bottom, andabrupt step, hydrodynamically increases fluid film pressure on theconvex surface of the shaft during rotation of the shaft.

In another embodiment of the present invention, the leading portionfurther defines an entrance to the pocket. Preferably, the sidebarsconverge along the rotation direction. The pocket hydrodynamicallygenerates pressure in the fluid that effects on the face a counteractionforce about the pivot and upstream relative to the rotation direction inorder to counteract the spragging force. This decreases oscillationtendencies of the true-tilting pad about the pivot.

In another embodiment, a method of bearing a journal dimensions thepocket to hydrodynamically increase pressure on the convex surface ofthe journal during rotation of the journal.

In yet another aspect of the present invention, first and secondtrue-tilting pads are adapted to cooperate with each other, a number ofother pads, and the fluid to hydrodynamically and mechanically supportthe shaft. The shaft tends during rotation to orbit in a pathapproximating an ellipse, but the hydrodynamically increasing pressuresare further for decreasing one or more of the axes of the ellipse. Also,this decreasing sizes the axes toward having substantially equal length.

In a further aspect of the invention, the first and second true-tiltingpads and the number of other pads are positioned to form an overallbearing having a substantially circular shape with a center. Thetrue-tilting pads tilt free of mechanical engagement with each other andwith the number of other pads. The hydrodynamically increasing pressuresare further for hydrodynamically centering the shaft at the center ofthe substantially circular shape of the overall bearing, during therotation.

In yet another embodiment, a method of bearing a journal dimensions thepocket of the first and second true-tilting pads and relativelypositions these true-tilting pads and the number of other pads toincrease support for the journal.

In a still further embodiment of the present invention, the true-tiltingpad is pivotally supported by a hydrostatic pivot, as well as leadingand trailing cam pivots that are fixedly connected to the housing.Additionally, the true-tilting pad has a backside and a body. Thebackside defines a cavity. The body defines a conduit between the pocketand the cavity. The leading and trailing portions are formed for tiltguidance with respect to the convex surface of the shaft by therespective leading and trailing cam pivots. During the rotation, thepocket hydrodynamically increases pressure on the convex surface of theshaft and hydrostatically increases pressure on an inner surface of thehousing. In addition, the body can define one or more passageways influid communication with the conduit for hydrostatic support of one ormore of the leading and trailing portions with respect to the campivots.

The present invention advantageously minimizes the occurrences ofunloaded pad instability and the effects of pad wiping over the pivot aswell as hot fluid carryover. Further, the present invention increaseshydrodynamic support for the convex surface of the shaft and decreaseshorsepower loss.

Additional features and advantages are realized through the structuresand techniques of the present invention. Other embodiments and aspectsof the invention are described in detail herein.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features, andadvantages of the invention will be apparent from the following detaileddescription of preferred embodiments taken in conjunction with theaccompanying drawings in which:

FIG. 1 depicts one example of a true-tilting pad of the presentinvention;

FIG. 2 depicts one example of an overall bearing using the true-tiltingpad of FIG. 1, in accordance with the principles of the presentinvention;

FIG. 3 plots a curve of eccentricity versus Sommerfeld number (whichincreases with decreasing load) for the true-tilting pad of FIG. 1, andsuch a curve for a conventional tilting pad;

FIG. 4 depicts one embodiment of the true-tilting pad of FIG. 1 suitablefor use with a hydrostatic pivot, in accordance with the principles ofthe present invention; and

FIG. 5 depicts one embodiment of the true-tilting pad of FIG. 1 wherethe leading portion does not define an entrance to the pocket, inaccordance with the principles of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In accordance with the principles of the present invention, ahydrodynamic and mechanical support capability is provided in which atrue-tilting pad of a journal bearing has its face toward a convexsurface of a shaft. The true-tilting pad tilts free of mechanicalengagement with one or more other pads. The face defines a pocket withlongitudinal sidebars, a bottom, and an abrupt step thathydrodynamically increase support for the convex surface, stabilize thetrue-tilting pad, and decrease horsepower loss, as described below.

One example of a true-tilting pad incorporating and using thehydrodynamic and mechanical support capability of the present inventionis depicted in FIGS. 1 and 2, and described in detail below.

In this example, true-tilting pad 100 includes a body 101, a face 102,and leading portion 104 as well as trailing portion 106. Thetrue-tilting pad exhibits little or no cross-coupling, as discussedbelow. As shown in FIG. 2, the leading portion is positioned upstreamrelative to rotation direction 108 of journal or shaft 110. Also, thetrailing portion is positioned downstream relative to the rotationdirection of the shaft. The face 102 extends longitudinally between theleading and trailing portions. The shaft is rotatable by prime mover 109and includes convex surface 111.

As depicted in FIG. 1, face 102 has border region 112 with engagementsurface 114. In the exemplary embodiment depicted, the border region iscoextensive with the engagement surface. Also, the border region definespocket 116 having longitudinal sidebars 121, 123, bottom 119, and abruptstep 120. The angle of the abrupt step with respect to the face 102 isin the range 89 to 115 degrees, and is most preferably in the range 89to 91 degrees. In one preferred embodiment, the sidebars converge alongthe rotation direction 108. This convergence reduces bulk proximitybetween the convex surface 111 and the face at entrance 118, andtherefore reduces horsepower loss.

As shown in FIG. 2, true-tilting pad 100 is pivotally supported by pivot122. Housing 124 supports the pivot and holds lubricating fluid 126,which can be gaseous or liquid. The true-tilting pad cooperates with oneor more other pads 127, which additional pads could be formed inaccordance with the present invention or conventionally. Thetrue-tilting pad tilts free of engagement with the other pads. Moreover,the true-tilting pad 100 and the other pads cooperate with each otherand the fluid to hydrodynamically and mechanically increase support forshaft 110. In one particular aspect of the present invention, the pocket116 of true-tilting pad 100 hydrodynamically increases pressure onconvex surface 111 during rotation of the shaft.

In the embodiment of the present invention depicted in FIG. 1, leadingportion 104 defines entrance 118 of pocket 116. Furthermore, borderregion 112 defines the pocket laterally interiorly with respect to face102.

Abrupt step 120 hydrodynamically constricts flow of fluid 126 during therotation of shaft 110. Entrance 118 and sidebars 121, 123 feed the fluidfor flow through pocket 116 and over the abrupt step 120. That is, theabrupt step constitutes an abrupt change that forces the streamlines ofthe fluid to dramatically converge. This abrupt restriction builds up alarge pressure on the face 102 upstream, relative to the rotation of theshaft, of the abrupt step 120. The abrupt step hydrodynamically enhancesthe wedge effect. Therefore, the fluid 126 flows relatively fasterbetween the abrupt step 120 and the convex surface 111 than immediatelyupstream. This yields relatively increased pressure immediately upstreamof the abrupt step 120. Accordingly, the pocket 116 hydrodynamicallygenerates additional lift on the convex surface 111. In one embodiment,this hydrodynamic lift is directed substantially along relative liftdirection 128, as depicted in FIG. 2.

Rotation of shaft 110 in the presence of fluid 126 naturally develops aspragging force about pivot 122. In particular, the rotation causesengagement between the fluid and face 102. This spragging force isdirected downstream relative to rotation direction 108 of the shaft.Advantageously, pocket 116 generates pressure in the fluid that effectson face 102 a counteraction force about the pivot and upstream relativeto the rotation direction. This counteraction force of the presentinvention desirably counteracts the spragging force. So, the pocket 116hydrodynamically produces a load that stabilizes true-tilting pad 100.In producing the counteraction force, the present inventionadvantageously discourages development of a couple moment about thepivot. Accordingly, the present invention decreases oscillationtendencies of the true-tilting pad about the pivot. This preventsspragging as well as babbitt metal failure.

Engagement surface 114, in one embodiment, is shaped concavely forsubstantially radial alignment with the convex surface of the shaft.Furthermore, in a preferred embodiment, pocket 116 longitudinallyextends over pivot 122 and thereby increases radial clearance of theconvex surface. So, the engagement surface 114 does not extend overpocket 116. Therefore, the decreased surface area of the engagementsurface 114 exposed for mechanical engaging with the convex surface 111,of the shaft 110, advantageously translates into decreased frictionduring mechanical engaging with the shaft. Namely, the convex surfaceonly wipes the engagement surface 114.

By reducing bulk proximity, one also decreases shearing stress andtemperature rise in fluid 126 located between the convex surface andface 102. Moreover, as discussed further below, one could varyperformance characteristics of true-tilting pad 100 to halve theshearing stress and thereby halve the horsepower loss.

With a typical input temperature of fluid 126 at about 120 degreesfahrenheit, the conventional tilting pad usually yields an output fluidtemperature of about 170 degrees fahrenheit. As outlined below, onecould vary the performance characteristics to yield instead an outputfluid temperature of about 145 degrees fahrenheit, following the same120 degrees fahrenheit input fluid temperature. Alternatively, thetrue-tilting pad could begin rather with an input fluid temperature ofabout 145 degrees to yield the same 170 degrees fahrenheit output fluidtemperature. This desirably allows use of a less viscous lubricant asthe fluid and accordingly reduces horsepower loss. Therefore, thetrue-tilting pad advantageously addresses the root cause of hot fluidcarryover by decreasing the increase in temperature of the fluid carriedover.

By reducing thermal gradients throughout the fluid, the true-tilting pad100 with abruptly-stepped pocket 116 lessens stresses in the pad itself.Many additional benefits also result from the decrease in fluidtemperature increase. For example, in known constructions of tiltingpads, the present invention advantageously lessens stresses at theinterface bond of the steel backing with the babbitt metal. Also, thepresent inventions minimizes silver removal, so different known mixturesof oil and additives can be used with various combinations of metals andalloys.

Detailed three-dimensional finite element analysis shows true-tiltingpad 100 can support the same load on shaft 110 as the conventionaltilting pad, while consuming only forty-five percent as much horsepower.The performance characteristics resulting from abrupt step 120 depend onparameters such as abrupt step clearance 129, abrupt step height 131,abrupt step width 130, abrupt step arc length 132, pad width 134, padarc length 136, and pivot clearance 138, as shown in FIGS. 1 and 2. Inparticular, the abrupt step clearance is the local fluid thickness asmeasured from the convex surface 111 to the face 102. Furthermore, thepivot clearance is measured from the convex surface to bottom 119 of thepocket 116. Desirably, one can develop a wide range of performancecharacteristics for the true-tilting pad by varying and tuning theseparameters.

In accordance with one preferred embodiment of the present invention,true-tilting pad 100 has the following characteristics. The ratio ofabrupt step clearance 129 to pivot clearance 138 is in the range 1.25 to10.00, and is most preferably in the range 1.80 to 2.20. Also, the ratioof abrupt step arc length 132 to pad arc length 136 is in the range 0.40to 0.85, and is most preferably in the range 0.42 to 0.75. Additionally,the ratio of abrupt step width 130 to pad width 134 is in the range 0.10to 0.95, and is most preferably in the range 0.30 to 0.90. Moreover,pivot 122 is preferably positioned longitudinally from leading edge 104approximately sixty percent along the pad arc length 136. Further, thepivot is positioned transversely symmetrically with respect to the padwidth. These optimizations enhance the converging wedge in fluid 126between face 102 and convex surface 111. This, in turn, reduces thehorsepower loss.

FIG. 2 shows an exemplary embodiment of overall journal bearing 300formed through selective positioning of five pads 100, 127, 140, 142,144. In one preferred embodiment, the pads are true-tilting padsconfigured in accordance with the present invention. The pads cooperatewith each other and fluid 126 to hydrodynamically increase pressures onconvex surface 111. One can tune the parameters of the pads to use thesepressures for hydrodynamically squeezing the convex surface during therotation. Further, the pads hydrodynamically generate increased lift onthe convex surface. Preferably, the overall bearing has a substantiallycircular shape with a center. These pressures advantageously canhydrodynamically center shaft 110 at the center of the substantiallycircular shape of the overall bearing, during the rotation.

Moreover, the present invention also addresses the problem of shaft 110tending to orbit elliptically during the rotation. In particular, thehydrodynamic pressures contributed by pads 100, 127, 140, 142, 144 canreduce, and size similarly, the major and minor axes of the ellipse.This shifts the tendency of the shaft toward rotating in a more-centeredand less-elliptical orbit.

In the case of shaft 110 having horizontal axis orientation, one couldselect pads 100, 127, 140, 142, 144 to be true-tilting pads and tunethem as follows. In order to reduce horsepower loss, one could selectarc length 136 of upper tilting pads 100, 127 to be half that of bottomtilting pad 142. Furthermore, one could reduce the pad width 134 of theupper tilting pads relative to that of the bottom tilting pad.Additionally, one could slightly reduce the arc length of side tiltingpads 140, 144, relative to the arc length of the bottom tilting pad. Asdescribed above, hydrodynamic loading stabilizes the upper pads againstspragging. Also, the side pads 140, 144 hydrodynamically squeeze theshaft to a more symmetrical and circular orbit. Further, the largehydrodynamic lift decreases load on the bottom pad. Decreasedeccentricity and decreased horsepower loss result. Mathematically, theoff-diagonal terms are zero, or nearly zero, in the stiffness anddamping matrices, as represented below, for overall bearing 300.

Spring Coefficients: (lb/in) ##EQU4## Damping Coefficients: (lb-sec/in)##EQU5## Therefore, there is little or no cross-coupling.

So, true-tilting pad 100, truly tilting because it exhibits little or nocross-coupling, suits use in all high performance turbo-equipment.Conversely, using the conventional interleaved tilting pads arrangement,and thereby introducing leading and trailing forces that make theoff-diagonal terms large, compromises performance and results ininstability.

For a given less than very-heavy load, a given speed, and a givengeometry, entrance 118, sidebars 121, 123, bottom 119, and abrupt step120 of pocket 116 produce a hydrodynamic force far in excess of theconventional tilting pad. In FIG. 3, a Sommerfeld number less than 0.1represents a very-heavy load. Moreover, eccentricity increases along thevertical axis; but, load decreases along the horizontal axis and soincreasing Sommerfeld numbers represent a decreasing load.

FIG. 3 depicts plots of eccentricity versus load for a conventionaltilting pad, curve 146, and the true-tilting pad of the presentinvention, curve 148. For purposes of illustration, curve 148corresponds to a true-tilting pad 100 formed with exemplary parametervalues, which by no means represent the absolute limit of performancefor other embodiments of the present invention formed having otherparameter values.

As illustrated in FIG. 3, true-tilting pad 100 of the present inventionproduces a much larger restoring force, and accordingly could supportmore load up to heavy loads, than the conventional tilting pad. Also,for a given less than very-heavy load, the true-tilting pad of thepresent invention yields much less eccentricity than the conventionaltilting pad. Moreover, on this plot, all solutions for the conventionaltilting pad lie along curve 146. Conversely, below intersection 149 ofcurves 146, 148, curve 148 only represents a limit of performance forthis particular embodiment of the present invention formed withexemplary parameter values. Further, the intersection 149 represents alimitation of performance of this embodiment of true-tilting pad 100only when bearing very-heavy loads.

That is, true-tilting pad 100 provides increased tunability for overallbearing 300. Tuning of parameters 129, 130, 132, 134, 136, 138 forinfluencing the fluid wedge, as well as of the positioning of pads 100,127, 140, 142, 144, allows one to employ superior lift to suit manypossible needs, uses, and environments. In accordance with the presentinvention, one can tune the parameters and positioning to suit anydesired characteristics bounded by curves 146, 148 below theirintersection 149. Namely, any solution below this intersection andbetween the bounds of the conventional pad and the limit for thisexemplary embodiment of the true-tilting pad of the present inventionrepresents a valid solution, in accordance with the present invention.Therefore, the true-tilting pad presents many more design parameters,heretofore unavailable, for working to meet specifications in multipleapplications of the present invention.

Numerous embodiments in accordance with the present invention work wellfor either horizontal or vertical axis orientation of shaft 110. Still,in accordance with the present invention, one can optimally tune theparameters and positioning to suit any particular axis orientation ofthe shaft. For instance, in a case of horizontal orientation of the axisof the shaft, curve 148 shows negative eccentricity for some lightloads, meaning one could tune the parameters and positioning tohydrodynamically force the center of the shaft vertically above thecenter of overall bearing 300. In particular, FIG. 3 represents a lightload by a Sommerfeld number greater than one.

As discussed above, the enhanced hydrodynamic force results fromadditional hydrodynamic pressure on convex surface 111 provided bypocket 116 on face 102 of true-tilting pad 100. The squeezing aspect ofthis enhanced hydrodynamic force can be represented mathematically by anincrease in magnitude of a transverse component of the force vectorcorresponding to the lift. In the case of vertical orientation of theshaft, one can give equivalent treatment to the components of the forcevector.

But, in the case of horizontal orientation of the shaft, one must treatthe components of the force vector dissimilarly. Namely, one typicallyaligns one component vertically, to be collinear with the influence ofgravity. In one embodiment, one can direct the enhancement capability ofthe present invention toward greatly increasing the remaining,horizontal component. Hydrodynamically, this tremendous increase in thehorizontal component contributes a large vertical lift, to combat thedeleterious effects on shaft 110 of the pull of gravity downward fromthe center of overall bearing 300. Nevertheless, the hydrodynamic liftenhancement also directly increases the vertical component of the forcevector.

True-tilting pad 100 of the present invention advantageously workstoward centering shaft 110 in clearance space 150 of overall bearing300, as depicted in FIG. 2. Therefore, pocket 116 serves to increase theefficiency of machinery. For example, in a turbine embodiment of theinvention, centering the shaft in the overall bearing advantageouslywould center the impeller in its annulus. During rotation of the shaft,pocket 116 desirably minimizes the eccentricity of the shaft positionand the ellipticity of its orbit, in accordance with the presentinvention.

In another embodiment in accordance with the present invention, entrance152, bottom 153, and abrupt step 154 of pocket 168 of true-tilting pad156 improve performance of a hydrostatic pivot 158, as illustrated inFIG. 4. The hydrostatic pivot includes feedhole 160, in the pocket 168of the face and body 161 of the pad, for feeding fluid 126 down tocavity 162 on backside 164 of the true-tilting pad. This works to floatthe true-tilting pad over inner surface 166 of housing 124. In oneembodiment, the hydrostatic pivot can operate with trailing and leadingcam pivots 167, 180 stationary on the housing, for engagement withcooperatively formed trailing and leading ends 170, 172, respectively,of the pad. The trailing and leading cam pivots 167, 180 accordinglyguide proper tilt of the true-tilting pad with respect to convex surface111 of shaft 110. Hydrostatic support in the present invention withrespect to trailing and leading cam pivots 167, 180 is discussed furtherbelow.

The higher pressures, thoroughly discussed above, produced by pocket 168can also increase pressure on fluid 126 about feedhole 160 of thehydrostatic pivot, thereby improving the floating performance ofhydrostatic pivot 158 as well. In particular, a conduit formed from thepocket 168, the feedhole 160, and the cavity 162 permits communicationof the fluid. So, one can tune parameters 129, 130, 132, 134, 136, 138to further produce high pressure on the fluid in the conduit. Thisadvantageously increases the hydrostatic pressure on the true-tiltingpad 156 in a direction away from inner surface 166 of housing 124.

In the particular embodiment depicted in FIG. 4, body 161 and trailingportion 170 define passageway 173 and cavity 174 for hydrostatic supportof the trailing portion 170 in a direction away from the trailing campivot 167. In accordance with the present invention, the passageway 173and the cavity 174 are in fluid communication with the feedhole 160.Therefore, the increased hydrostatic pressure in the feedhole 160,according to the present invention, advantageously increases hydrostaticpressure on the trailing portion 170 with respect to trailing cam pivot167. In addition, body 161 and leading portion 172 can define passageway176 and cavity 178, in fluid communication with the feedhole 160, forhydrostatic support of the leading portion 172 with respect to leadingcam pivot 180. Furthermore, one can tune parameters 129, 130, 132, 134,136, 138 to produce desired pressures on the fluid in these passageways173, 176 and respective cavities 174, 178 for hydrostatic support of thetrailing and leading portions 170, 172.

In the embodiment of the present invention depicted in FIG. 5, leadingportion 104 does not define an entrance to pocket 116.

Numerous alternative variations exist for practicing the presentinvention. One could easily vary the number and arrangement of pads. Forinstance, one could easily form overall bearing 300 using instead aconventional pad along with two true-tilting pads configured inaccordance with the present invention. Of course, border region 112 neednot be coextensive with engagement surface 114. Further, engagementsurface 114 could be rippled, dimpled, or diamonded and need nototherwise be continuous. Longitudinal sidebars 121, 123 can easily bealigned in parallel or have various oblique alignments. Naturally,entrance 118 to pocket 116 could be defined forward of leading region104, such as by gradations on face 102. In the event hydrostatic pivot158 uses a feedhole bypassing pocket 168, one can further define thepocket to include a passageway to the feedhole. Then, the passageway andthe feedhole together form the conduit between the pocket and cavity162.

Although preferred embodiments have been depicted and described indetail herein, it will be apparent to those skilled in the relevant artthat various modifications, additions, substitutions and the like can bemade without departing from the spirit of the invention and these aretherefore considered to be within the scope of the invention as definedin the following claims.

What is claimed is:
 1. A method of bearing a journal, said method comprising;positioning a leading portion of a true-tilting pad upstream relative to a rotation direction of the journal and free of mechanical engagement with one or more other pads of a bearing for the journal; positioning a trailing portion of the true-tilting pad downstream relative to the rotation direction of the journal and free of mechanical engagement with the other pads; dimensioning a pocket, in forming the pocket on a face of the true-tilting pad extending between the leading and trailing portions, to hydrodynamically increase pressure on a convex surface of the journal during rotation of the journal; and directing said face toward the journal.
 2. The method of claim 1, further comprising dimensioning the pocket to hydrodynamically and mechanically decrease an amount of horsepower expended to rotate the journal during the rotation of the journal.
 3. The method of claim 1, further comprising dimensioning the pocket to hydrodynamically generate pressure that effects on the face a counteraction force about a pivot of the true-tilting pad and upstream relative to the rotation direction in order to counteract a spragging force.
 4. The method of claim 1, further comprising dimensioning the pocket to hydrodynamically generate a load on the face that decreases oscillation tendencies of the true-tilting pad.
 5. A method of bearing a journal, said method comprising:providing first and second true-tilting pads and a number of other pads to bear the journal; positioning a leading portion of the first true-tilting pad upstream relative to a rotation direction of the journal; positioning a trailing portion of the first true-tilting pad downstream relative to the rotation direction of the journal; positioning a leading portion of the second true-tilting pad upstream relative to the rotation direction of the journal; positioning a trailing portion of the second true-tilting pad downstream relative to the rotation direction of the journal; positioning the leading and trailing portions of the first true-tilting pad free of mechanical engagement with the second true-tilting pad and with the number of other pads; positioning the leading and trailing portions of the second true-tilting pad free of mechanical engagement with the first true-tilting pad and with the number of other pads; dimensioning a pocket in forming the pocket on a face of the first true-tilting pad extending between the leading and trailing portions of the first true-tilting pad, dimensioning a pocket in forming the pocket on a face of the second true-tilting pad extending between the leading and trailing portions of the second true-tilting pad, relatively positioning the first and second true-tilting pads and the number of other pads, and directing said faces toward the journal to increase support for the journal.
 6. The method of claim 5, wherein said dimensioning of pockets of said first and second true-tilting pads and said relatively positioning said first and second true-tilting pads and said number of other pads hydrodynamically generates increased lift on the convex surface of the journal during the rotation of the journal.
 7. The method of claim 5, wherein said dimensioning of pockets of said first and second true-tilting pads and said relatively positioning said first and second true-tilting pads and said number of other pads hydrodynamically provide a decreased-ellipticity path of orbiting of the journal during the rotation of the journal.
 8. The method of claim 5, wherein relative positions of the first and second true-tilting pads and the number of other pads form an overall bearing having a substantially circular shape with a center; and wherein said dimensioning of pockets of said first and second true-tilting pads and said relatively positioning said first and second true-tilting pads and said number of other pads hydrodynamically promote centering of the journal toward the center of the substantially circular shape of the overall bearing, during the rotation of the journal.
 9. The method of claim 5, wherein said dimensioning of pockets of said first and second true-tilting pads and said relatively positioning said first and second true-tilting pads and said number of other pads hydrodynamically provide an increased-symmetry path of orbiting of the journal during the rotation of the journal.
 10. The method of claim 5, wherein said dimensioning of pockets of said first and second true-tilting pads and said relatively positioning said first and second true-tilting pads and said number of other pads hydrodynamically promote symmetry of orbiting of the journal during the rotation of the journal. 